1. Field of the Invention
The invention relates to processes for preparing synthetic polymeric resins under molecular weight controlling conditions and more particularly relates to processes for controlling the weight average molecular weight of poly-carbonates.
2. Brief Description of Related Art
The preparation of synthetic polymeric resins by the condensation or step-reaction polymerization of one or more monomer reactants is well known and includes the preparation of polycarbonate resins. Generally, the degree of polymerization is controlled by the proportion of at least one reactant in the polymerization. Often the controlling reactant is a compound which, when incorporated into the polymerization reaction, terminates the forming polymer chain. Such compounds are sometimes referred to as "endcappers" or "chain-stoppers." The compounds are in fact molecular weight regulators. Unfortunately, these molecular weight regulators react in the polymerization randomly, i.e., they will terminate chain length propagation haphazardly and not predictably so that the resin products are mixtures of polymer chains of various and random lengths (molecular weights).
By fixing the stoichiometry and reaction conditions, typically in batch preparations, one can usually come close to reproducing batches which are substantially alike in terms of a weight average molecular weight (M.sub.w). However, even these batches show weight average molecular weight (M.sub.w) variation from each other. The problem of eliminating these variations is of commercial importance, since the end user of the resin requires uniformity in physical properties of molded products. Generally, specifications are to be met in terms of molecular weights (M.sub.w).
Those skilled in the art will appreciate the commercial importance of preparing synthetic polymeric resins, which are consistently uniform in their physical properties (especially in regard to weight average molecular weight). One example of an effort to achieve this goal (in respect to polycarbonate preparation) is described in the U.S. Pat. No. 3,240,755 (Cawthon et al., 1966). This Patentee employed in the process fractional extraction with selected solvents which would separate polycarbonate resin chains of differing molecular weights. However, among the separated fractions, there remained considerable variations in the product weight average molecular weights.
The process of the present invention is an improvement in the art, particularly in respect to obtaining consistent weight average molecular weights in a resin product.
We have now found that the weight average molecular weight of a given polycarbonate resin can be controlled and made more homogeneous by addition of reactive multifunctional aliphatic compounds. The weight average molecular weight of batches of polycarbonate can be adjusted to a predetermined value.
Reactive multifunctional aliphatic compounds react with polycarbonate in a non-catalyzed reaction. The amount reacted determines the ultimate weight average molecular weight of the polycarbonate resin batch. The reactive aliphatic compound is converted to a harmless, unreactive, cyclic carbonate. The latter is demonstrated by subjecting the polycarbonate resin product to heat treatment. Heat treatments do not influence the M.sub.w of the product dramatically. The process of the invention can be used to tailor-make resins with specific weight average molecular weight. This would enable one to reduce the inventory of linear polycarbonate resins produced by the interfacial process.
One of the difficulties, prior to this invention, in achieving consistent, narrow weight average molecular weight ranges in the preparation of polycarbonates is related to impurities in the dihydric phenols (I) described above. For example, bisphenol A of even the highest purity includes contaminant compounds which can act as molecular weight regulators (chain stoppers) or their equivalents. Representative of such equivalents are o, p'-bisphenol A, chroman I, spirobindane (6, 6.sup.1 -dihydroxy-3,3,3',3'-tetramethylspiro(bis)indane) and the like which are normally present, in varying quantities, as impurities associated with dihydric phenols like bisphenol A. It is difficult to take into consideration their presence, at the beginning of polymerization because of the variability of their presence. However, they can be accounted for as molecular weight controlling factors during polymerization according to the process of the present invention.